Computing devices, such as notebook computers, personal data assistants (PDAs), kiosks, and mobile handsets, have user interface devices, which are also known as human interface devices (HID). One user interface device that has become more common is a touch-sensor pad (also commonly referred to as a touchpad). A basic notebook computer touch-sensor pad emulates the function of a personal computer (PC) mouse. A touch-sensor pad is typically embedded into a PC notebook for built-in portability. A touch-sensor pad replicates mouse X/Y movement by using two defined axes which contain a collection of sensor elements that detect the position of one or more conductive objects, such as a finger. Mouse right/left button clicks can be replicated by two mechanical buttons, located in the vicinity of the touchpad, or by tapping commands on the touch-sensor pad itself. The touch-sensor pad provides a user interface device for performing such functions as positioning a pointer, or selecting an item on a display. These touch-sensor pads may include multi-dimensional sensor arrays for detecting movement in multiple axes. The sensor array may include a one-dimensional sensor array, detecting movement in one axis. The sensor array may also be two dimensional, detecting movements in two axes.
Another user interface device that has become more common is a touch screen. Touch screens, also known as touchscreens, touch windows, touch panels, or touchscreen panels, are transparent display overlays which are typically either pressure-sensitive (resistive or piezoelectric), electrically-sensitive (capacitive), acoustically-sensitive (surface acoustic wave (SAW)) or photo-sensitive (infra-red). The effect of such overlays allows a display to be used as an input device, removing the keyboard and/or the mouse as the primary input device for interacting with the display's content. Such displays can be attached to computers or, as terminals, to networks. Touch screens have become familiar in retail settings, on point-of-sale systems, on ATMs, on mobile handsets, on kiosks, on game consoles, and on PDAs where a stylus is sometimes used to manipulate the graphical user interface (GUI) and to enter data. A user can touch a touch screen or a touch-sensor pad to manipulate data. For example, a user can apply a single touch, by using a finger to touch the surface of a touch screen, to select an item from a menu.
A certain class of touch sense arrays includes a two-dimensional array of capacitors, referred to as sense elements. Touch sense arrays can be scanned in several ways, one of which (mutual-capacitance sensing) permits individual capacitive elements to be measured. Another method (self-capacitance sensing) can measure an entire sensor strip, or even an entire sensor array, with less information about a specific location, but performed with a single read operation.
The two-dimensional array of capacitors, when placed in close proximity, provides a means for sensing touch. A conductive object, such as a finger or a stylus, coming in close proximity to the touch sense array causes changes in the capacitances of the sense elements in proximity to the conductive object. More particularly, when a finger touch occurs, self-capacitance increases whereas mutual capacitance decreases. These changes in capacitance can be measured to produce a “two-dimensional map” that indicates where the touch on the array has occurred.
One way to measure such capacitance changes is to form a circuit comprising a signal driver (e.g., an AC current or a voltage source) which is applied to each horizontally aligned conductor in a multiplexed fashion. The charge associated with each of the capacitive intersections is sensed and similarly scanned at each of the vertically aligned electrodes in synchronization with the applied current/voltage source. This charge is then measured using a touch sense controller that typically includes a form of charge-to-voltage converter, followed by a multiplexer and A/D converter that is interfaced with a central processing unit (CPU) to convert the input signal to digital form. The CPU, in turn, renders the “two-dimensional map” or “touch map” and determines the location of a touch. A full scan of a panel of pairs of electrodes (i.e., slots) is performed before the resulting data is processed by the CPU in a serial fashion. Unfortunately, such “serial” scanning is too slow for modern touch screen devices.
An alternative approach is to perform capacitance-to-code conversion (scanning) using dedicated blocks of digital logic (i.e., a HW scanning engine) and the converted data is processed by a CPU. Unfortunately, such a hardware-based scanning engine is implemented in synthesized digital logic blocks with fixed implementation (i.e., having non-programmable elements). As a result, the hardware-based scanning engine operates according to algorithms defined at design time of the integrated circuit (IC). Advanced features such as adaptive scanning algorithms that require special timing, additional control signals, and/or extra measurement cycles require redesign of the integrated circuit (IC) on which hardware-based scanning engine is fabricated.
Further, high conversion speeds may not be attainable because CPU processing is frequently interrupted by the hardware-based scanning engine when RX and TX multiplexers have to be reconfigured between conversions. This reduces data reporting rates.